I would disagree with passively 'letting land return to nature'. If you did this in Australia (where I'm from), and likely other places too, you'd end up with a haven for invasive species, which actively damage surrounding ecosystems.

Land needs to be actively managed - if you want it in its natural state, it needs time, money, and effort.

In my (biased) opinion, the best thing we can do for the planet is to spend more on research, so we can understand and counter problems. Spending on agricultural science is falling world-wide, just when it's needed the most. That isn't good.